The town of Frankfurt will hold a bicycle rally tomorrow, a large event for the German city, and one expected to get international coverage. According to German prosecutors, a husband-and-wife team planned to ensure that attention. A raid last night resulted in their arrest on terrorism charges, and the discovery of a very large cache of bomb-making materials:

Frankfurt prosecutor Albrecht Schreiber said that a 35-year-old man and his 34-year-old wife were detained on suspicion of plotting a “severe criminal act.” A pipe bomb, parts of a bazooka and an assault rifle, and about 100 bullets were found in their home — along with diesel fuel and three liters of hydrogen peroxide.

Those items “could be used for the building of a bomb,” Schreiber added, saying the couple also had several “conservative religious texts” in their apartment.

The prosecutor said the two had been monitored by German police for “quite a while” and that the timing of the raid was influenced by the suspect’s surveillance of the area where a bicycle race was due to come through the town on Friday.

Police in southern Germany have prevented a planned Islamist attack, a German newspaper reported today, adding that a married couple had been arrested overnight after weapons and explosives were found in their home. …

The Die Welt newspaper only identified the couple by their first names and initial – Halil and Senay D. The paper’s website said Halil had links to the Salafist scene in Frankfurt and the al Qaeda network. It did not identify the source for its report.

Police began surveillance after the couple bought large quantities of chemicals that could be used to make home-made bombs and started using false names, Die Welt said.

Global News reports that the decision to finally arrest the pair came after the husband staked out a forest along the route of the bicycle rally:

The German newspaper report said police decided to move in after one of the suspects was observed exploring a forest near Oberursel where a bicycle race is expected to take place May 1.

The Associated Press has more, including an assertion that the man was a “chemistry student.” The amount of chemicals purchased first attracted the attention of authorities, and their use of false identities raised suspicions even further. At least from the broad outlines of the details released by prosecutors earlier, it appears that this couple intended on conducting a Boston Marathon-style attack — although the forest location is a little odd. Perhaps they suspected that the police would be looking for trouble at the start and end of the race and planned to conduct the attack where they thought security would be weakest.

It’s also a little unusual for an AQ-linked operation, which usually involves transportation and/or government buildings and multiple attack points. Over the last few years, though, AQ and especially its Yemeni arm AQAP have aimed at softer targets — or this could be another of the “known wolf” operations that have popped up more often. The good news is that German authorities found out about this plot. The potential bad news is that we don’t know whether the kind of red flag that alerted them to this couple will get raised on the next one.